Semiconductor memory device

ABSTRACT

A semiconductor memory device in accordance with an embodiment comprises first lines, second lines, and a memory cell array including memory cells. Each of the memory cells is disposed at each of intersections of the first lines and the second lines and is configured by a rectifier element and a variable resistor connected in series. The rectifier element comprises a first semiconductor region of a first conductivity type including an impurity of a first impurity concentration, and a second semiconductor region of a second conductivity type including an impurity of a second impurity concentration lower than the first impurity concentration. The first semiconductor region and the second semiconductor region are formed by silicon. A junction interface of the first semiconductor region and the second semiconductor region is a pseudo-heterojunction formed by two layers that have different band gap widths and are formed of the same material.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2010-164274, filed on Jul. 21,2010, the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Field

Embodiments described in this specification relate to a semiconductormemory device comprising an arrangement of memory cells, the memorycells configured to store data by change in a resistance value of avariable resistor.

2. Description of the Related Art

In recent years, along with a rising level of integration insemiconductor devices, circuit patterns of transistors and so onconfiguring these semiconductor devices are being increasinglyminiaturized. Required in this miniaturization of the patterns is notsimply a thinning of line width but also an improvement in dimensionalaccuracy and positioning accuracy of the patterns. This trend appliesalso to semiconductor memory devices.

Conventionally known and marketed semiconductor memory devices such asDRAM, SRAM, and flash memory each use a MOSFET as a memory cell.Consequently, there is required, accompanying the miniaturization ofpatterns, an improvement in dimensional accuracy at a rate exceeding arate of the miniaturization. As a result, a large burden is placed alsoon the lithography technology for forming these patterns which is afactor contributing to a rise in product cost.

In recent years, resistance varying memory is attracting attention as acandidate to succeed these kinds of semiconductor memory devicesemploying a MOSFET as a memory cell. The resistance varying memoryherein includes not only resistance varying memory (ReRAM: ResistiveRAM) in its narrow definition, which uses a transition metal oxide as arecording layer to store a resistance state of the transition metaloxide in a non-volatile manner, but also phase change memory (PCRAM:Phase Change RAM), which uses chalcogenide or the like as a recordinglayer to utilize resistance information of a crystalline state(conductor) and an amorphous state (insulator).

Write of data to a memory cell is performed by applying a certainvoltage to a variable resistor for a short time. This causes thevariable resistor to change from a high-resistance state to alow-resistance state. This operation to change the variable resistorfrom the high-resistance state to the low-resistance state is hereafterreferred to as a setting operation.

On the other hand, erase of data in a memory cell is performed byapplying a certain voltage to a variable resistor for a long time, thecertain voltage being lower than the voltage applied during the settingoperation, and the variable resistor being one in the low-resistancestate subsequent to the setting operation. This causes the variableresistor to change from the low-resistance state to the high-resistancestate. This operation to change the variable resistor from thelow-resistance state to the high-resistance state is hereafter referredto as a resetting operation. The memory cell adopts, for example, thehigh-resistance state as a stable state (reset state), and, in the caseof binary data storage, write of data is performed by the settingoperation in which the reset state is changed to the low-resistancestate.

During the resetting operation, a large current, which acts as aresetting current, must be passed through the memory cell. As a result,a diode connected in series to the variable resistor is required to havean output current which is large. However, if a simple PN junction diodeis used for the diode, an unselected memory cell cannot be applied witha voltage greater than a voltage determined by the junction breakdownvoltage of the PN junction diode, whereby the output current of the PNjunction diode is limited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a semiconductor memory device in accordancewith a first embodiment of the present invention.

FIG. 2 is a perspective view of part of a memory cell array 1.

FIG. 3 is a cross-sectional view of one memory cell taken along the lineI-I′ and seen in the direction of the arrows in FIG. 2.

FIG. 4 is a view showing an example of a variable resistor VR.

FIG. 5 is a view showing an example of a variable resistor VR.

FIG. 6 is a view showing a separate configuration example of the memorycell array 1.

FIG. 7 is a view showing the separate configuration example of thememory cell array 1.

FIG. 8 is a circuit diagram of the memory cell array 1 and peripheralcircuits thereof.

FIG. 9 is a cross-sectional view showing a structure of a memory cell inthe first embodiment.

FIG. 10 is a cross-sectional view showing a structure of a diode in acomparative example.

FIG. 11 is a view showing a state of energy bands in the diode in thecomparative example.

FIG. 12 is a view showing a state of energy bands in a diode in thefirst embodiment.

FIG. 13 is a cross-sectional view showing a structure of a diode inanother example.

DETAILED DESCRIPTION

A semiconductor memory device in accordance with an embodiment comprisesa plurality of first lines disposed in parallel, a plurality of secondlines disposed to intersect the first lines, and a memory cell arrayincluding memory cells. Each of the memory cells is disposed at each ofintersections of the first lines and the second lines, and each of thememory cells is configured by a rectifier element and a variableresistor connected in series. The rectifier element comprises a firstsemiconductor region of a first conductivity type including an impurityof a first impurity concentration, and a second semiconductor region ofa second conductivity type including an impurity of a second impurityconcentration lower than the first impurity concentration. The firstsemiconductor region and the second semiconductor region are formed bysilicon. A junction interface of the first semiconductor region and thesecond semiconductor region is a pseudo-heterojunction formed by twolayers that have different band gap widths and are formed of the samematerial.

Next, embodiments of the present invention are described with referenceto the drawings. Note that in the following descriptions of drawings inthe embodiments, identical symbols are assigned to places havingidentical configurations, and descriptions thereof are omitted.

[Overall Configuration]

FIG. 1 is a block diagram of a nonvolatile memory in accordance with afirst embodiment of the present invention. The nonvolatile memorycomprises a memory cell array 1 of memory cells arranged in a matrix,each memory cell including a later-described ReRAM (variable resistor).

A column control circuit 2 is provided at a position adjacent to thememory cell array 1 in a bit line BL direction. The column controlcircuit 2 controls the bit line BL in the memory cell array 1 to erasedata in the memory cell, write data to the memory cell, and read datafrom the memory cell. A row control circuit 3 is provided at a positionadjacent to the memory cell array 1 in a word line WL direction. The rowcontrol circuit 3 selects the word line WL in the memory cell array 1and applies voltages required to erase data in the memory cell, writedata to the memory cell, and read data from the memory cell.

A data input/output buffer 4 is connected to an external host 9 via aninput/output (I/O) line to receive write data, receive eraseinstructions, output read data, and receive address data and commanddata. The data input/output buffer 4 sends received write data to thecolumn control circuit 2 and receives read data from the column controlcircuit 2 and outputs it to external. An address supplied from externalto the data input/output buffer 4 is sent via an address register 5 tothe column control circuit 2 and the row control circuit 3.

A command supplied from the host 9 to the data input/output buffer 4 issent to a command interface 6. The command interface 6 receives anexternal control signal from the host 9 and decides whether the datainputted to the data input/output buffer 4 is write data, a command oran address. If the data is a command, then the command interface 6transfers it as a received command signal to a state machine 7.

The state machine 7 manages the entire nonvolatile memory to receivecommands from the host 9 via the command interface 6, and perform read,write, erase, data input/output management, and so on. The external host9 can also receive status information managed by the state machine 7 anddecide the operation result. The status information is also utilized incontrol of write and erase. Further, the state machine 7 controls apulse generator 8. Under this control, the pulse generator 8 is allowedto output a pulse of any voltage at any timing.

The pulse formed herein can be transferred to any line selected by thecolumn control circuit 2 and the row control circuit 3. Note thatperipheral circuit elements other than the memory cell array 1 can beformed in a silicon substrate immediately beneath the memory cell array1 formed in a wiring layer. Thus, the chip area of the nonvolatilememory can be made almost equal to the area of the memory cell array 1.

[Memory Cell Array and Peripheral Circuits]

FIG. 2 is a perspective view of part of the memory cell array 1, andFIG. 3 is a cross-sectional view of one memory cell taken along the lineI-I′ and seen in the direction of the arrows in FIG. 2. There are pluralword lines WL0-WL2 as first lines disposed in parallel, which intersectplural bit lines BL0-BL2 as second lines disposed in parallel. A memorycell MC is disposed at each of intersections of the word lines WL0-WL2and bit lines BL0-BL2 to be sandwiched therebetween. Desirably, thefirst and second lines are composed of a heat-resistive low-resistancematerial such as W, WSi, NiSi, or CoSi.

[Memory Cell MC]

As shown in FIG. 3, the memory cell MC comprises a serial connectioncircuit of a variable resistor VR and a diode DI. Used for the variableresistor VR is a substance which can vary its resistance throughcurrent, heat, or chemical energy on voltage application. Disposed aboveand below the variable resistor VR and the diode DI are electrodes EL1,EL2, and EL3 configured to function as a barrier metal layer and anadhesive layer. Disposed above the electrode EL1 is the variableresistor VR, and disposed above the variable resistor VR is theelectrode EL2. Disposed above the electrode EL2 is the diode DI, anddisposed above the diode DI is the electrode EL3. Material of theelectrodes EL1 and EL3 may include Pt, Au, Ag, TiAlN, SrRuO, Ru, RuN,Ir, Co, Ti, TiN, TaN, LaNiO, Al, PtIrO_(x), PtRhO_(X), Rh, TaAlN, W, orthe like. Moreover, material of the electrode EL2 may include W, WN,TaN, TaSiN, TaSi₂, TiC, TaC, Nb—TiO₂, or the like. Material of theelectrode EL2 may similarly be adopted also for the electrodes EL1 andEL3. In addition, a metal film capable of achieving uniform orientationmay also be interposed. Moreover, a separate buffer layer, barrier metallayer and adhesive layer may further be interposed.

[Diode DI]

As shown in FIG. 3, the diode DI is a PN junction diode comprising a p+type layer D1 (first semiconductor region), an n− type layer D2 (secondsemiconductor region), and an n+ type layer D3. The “+” and “-” symbolsused here indicate magnitude of impurity concentration. Configuration ofthe diode DI is described in detail later.

[Variable Resistor VR]

The variable resistor VR may include one that comprises a compositecompound containing cations of a transition element and varies theresistance through migration of the cations (ReRAM).

FIGS. 4 and 5 show examples of the variable resistor VR. The variableresistor VR shown in FIG. 4 includes a recording layer 12 arrangedbetween electrode layers 11, 13. The recording layer 12 is composed of acomposite compound containing at least two types of cation elements. Atleast one of the cation elements is a transition element having thed-orbit incompletely filled with electrons, and the shortest distancebetween adjacent cation elements is 0.32 nm or lower. Specifically, itis represented by a chemical formula A_(x)M_(y)X_(z) (A and M aredifferent elements) and may be configured by material having acrystalline structure such as a spinel structure (AM₂O₄), an ilmenitestructure (AMO₃), a delafossite structure (AMO₂), a LiMoN₂ structure(AMN₂), a wolframite structure (AMO₄), an olivine structure (A₂MO₄), ahollandite structure (A_(x)MO₂), a ramsdellite structure (A_(x)MO₂), anda perovskite structure (AMO₃).

Two types of configurations of a variable resistor in resistance varyingmemory are known. One type, known as a bipolar type, sets ahigh-resistance state and a low-resistance state by switching thepolarity of applied voltage. The other type, known as a unipolar type,sets the high-resistance state and the low-resistance state bycontrolling voltage value and voltage application time, withoutswitching the polarity of applied voltage.

The unipolar type is preferable for realizing a high density memory cellarray. This is because, in the case of the unipolar type, the cell arraycan be configured by stacking a variable resistor and a rectifierelement such as a diode at intersections of bit lines and word lines,without using transistors. Furthermore, arranging this kind of memorycell array to be stacked three-dimensionally makes it possible toachieve a large capacity, without increasing cell array area.

That is, adopting the serial connection circuit of a variable resistorVR and a diode DI for the memory cell MC allows execution of a settingoperation configured to apply a certain voltage to the variable resistorVR for a short time, and a resetting operation configured to apply acertain voltage, which is lower than the voltage applied during thesetting operation, to the variable resistor VR for a long time. Thepresent embodiment can be easily applied to the diode accompanying aunipolar type variable resistor.

In the example of FIG. 4, A comprises Zn, M comprises Mn, and Xcomprises O, ZnMn₂O₄ being used. Alternatively, the variable resistor VRmay also be configured by a thin film comprising one material from amongNiO, TiO₂, SrZrO₃, Pr_(0.7)Ca_(0.3)MnO₃, and the like.

In the recording layer 12 of FIG. 4, a small white circle represents adiffused ion (Zn), a large white circle represents an anion (O), and asmall black circle represents a transition element ion (Mn). The initialstate of the recording layer 12 is the high-resistance state. When theelectrode layer 11 is kept at a fixed potential and a negative voltageis applied to the electrode layer 13, part of diffused ions in therecording layer 12 migrate toward the electrode layer 13 to reducediffused ions in the recording layer 12 relative to anions. The diffusedions arrived at the electrode layer 13 accept electrons from theelectrode layer 13 and precipitate as a metal, thereby forming a metallayer 14. Inside the recording layer 12, anions become excessive andconsequently increase the valence of the transition element ion in therecording layer 12. As a result, the carrier injection brings therecording layer 12 into electron conduction and thus completes thesetting operation. On reading, a current may be allowed to flow, ofwhich value is very small so that the material configuring the recordinglayer causes no resistance variation. The programmed state(low-resistance state) may be reset to the erased state (high-resistancestate) by supplying a large current flow in the recording layer 12 for asufficient time, which causes Joule heating to facilitate the oxidationreduction reaction in the recording layer 12. Application of an electricfield in the opposite direction from that at the time of setting mayalso allow the resetting operation to be performed.

In the example of FIG. 5, a recording layer 15 sandwiched between theelectrode layers 11, 13 is formed of two layers: a first compound layer15 a and a second compound layer 15 b. The first compound layer 15 a isarranged on the side close to the electrode layer 11 and represented bya chemical formula A_(x)Ml_(y)Xl_(z). The second compound layer 15 b isarranged on the side close to the electrode layer 13 and has gap sitescapable of accommodating cation elements from the first compound layer15 a.

In the example of FIG. 5, A comprises Mg, Ml comprises Mn, and Xlcomprises 0 in the first compound layer 15 a. The second compound layer15 b contains Ti shown with black circles as transition reduction ions.Moreover, in the first compound layer 15 a, a small white circlerepresents a diffused ion (Mg), a large white circle represents an anion(O), and a double circle represents a transition element ion (Mn). Notethat the first compound layer 15 a and the second compound layer 15 bmay be stacked in multiple layers such as two or more layers.

In this variable resistor VR, potentials are given to the electrodelayers 11, 13 so that the first compound layer 15 a serves as an anodeand the second compound layer 15 b serves as a cathode to cause apotential gradient in the recording layer 15. In this case, part ofdiffused ions in the first compound layer 15 a migrate through thecrystal and enter the second compound layer 15 b on the cathode side.The crystal of the second compound layer 15 b includes gap sites capableof accommodating diffused ions. Accordingly, the diffused ions movedfrom the first compound layer 15 a are trapped in the gap sites.Therefore, the valence of the transition element ion in the firstcompound layer 15 a increases while the valence of the transitionelement ion in the second compound layer 15 b decreases. In the initialstate, the first and second compound layers 15 a, 15 b may be in thehigh-resistance state. In such a case, migration of part of diffusedions in the first compound layer 15 a therefrom into the second compoundlayer 15 b generates conduction carriers in the crystals of the firstand second compounds, and thus both have electrical conduction.

Note that the programmed state (low-resistance state) may be reset tothe erased state (high-resistance state) by supplying a large currentflow in the recording layer 15 for a sufficient time for Joule heatingto facilitate the oxidation reduction reaction in the recording layer15, similarly to the preceding example. Application of an electric fieldin the opposite direction from that at the time of setting may alsoallow resetting.

[Modified Example of Memory Cell Array]

Moreover, as shown in FIG. 6, plural such memory structures describedabove may be stacked to form a three-dimensional structure. FIG. 7 is across-sectional view showing a II-II′ section in FIG. 6. The shownexample relates to a memory cell array of a 4-layer structure havingcell array layers MA0-MA3.

A word line WL0 j is shared by an upper and a lower memory cell MC0,MC1. A bit line BL1 i is shared by an upper and a lower memory cell MC1,MC2. A word line WL1 j is shared by an upper and a lower memory cellMC2, MC3.

In place of the line/cell/line/cell repetition, an interlayer insulatingfilm may be interposed as aline/cell/line/interlayer-insulating-film/line/cell/line between cellarray layers. Note that the memory cell array 1 may be divided into MATsof several memory cell groups. The column control circuit 2 and the rowcontrol circuit 3 described above may be provided on a MAT-basis, asector-basis, or a cell array layer MA-basis or shared by them.Alternatively, they may be shared by plural bit lines BL to reduce thearea.

FIG. 8 is a circuit diagram of the memory cell array 1 and peripheralcircuits thereof. For simplicity, the description advances on theassumption that the memory has a single-layered structure. In FIG. 8,the diode DI contained in the memory cell MC has its anode connected tothe word line WL and its cathode connected to the bit line BL via thevariable resistor VR. Each bit line BL has one end connected to aselection circuit 2 a, which is part of the column control circuit 2.Each word line WL has one end connected to a selection circuit 3 a,which is part of the row control circuit 3.

The selection circuit 2 a includes a selection PMOS transistor QP0 and aselection NMOS transistor QN0, provided to each bit line BL, of whichgates and drains are commonly connected. The selection PMOS transistorQP0 has its source connected to a high potential power supply Vcc. Theselection NMOS transistor QN0 has its source connected to a bit-lineside drive sense line BDS, which is used to apply a write pulse andsupply a detection current during data read. The transistors QP0, QN0have a common drain connected to the bit line BL, and a common gatesupplied with a bit-line selection signal BSi for selecting each bitline BL.

The selection circuit 3 a includes a selection PMOS transistor QP1 and aselection NMOS transistor QN1, provided to each word line WL, of whichgates and drains are commonly connected. The selection PMOS transistorQP1 has its source connected to a word-line side drive sense line WDS,which is used to apply a write pulse and supply a detection currentduring data read. The selection NMOS transistor QN1 has its sourceconnected to a low potential power supply Vss. The transistors QP1, QN1have a common drain connected to the word line WL and a common gatesupplied with a word-line selection signal /WSi for selecting each wordline WL.

The example shown above is suitable for selecting the memory cellsindividually. In contrast, in batch read of data from plural memorycells MC connected to the word line WL1, sense amplifiers are arrangedindividually for the bit lines BL0-BL2, and the bit lines BL0-BL2 areconnected to the sense amplifiers individually via the selection circuit2 a using the bit-line selection signals BS. In addition, the memorycell array 1 may be configured to include diodes DI having a polarityreversed from that of the circuit shown in FIG. 7 (connected having adirection from the bit line BL to the word line WL as a forward biasdirection), such that current flows from the bit line BL side to theword line WL side.

[Diode DI]

Next, a configuration of the diode DI in the memory cell MC is describedin detail with reference to FIG. 9. FIG. 9 is a cross-sectional viewshowing a structure of the memory cell MC and the diode DI in accordancewith the embodiment. As previously mentioned, the memory cell MC isconfigured by the diode DI, variable resistor VR, and the metalelectrodes EL1-EL3 connected in series.

As shown in FIG. 9, the diode DI in accordance with the embodiment is aPN junction diode comprising the p+ type layer D1, the n− type layer D2,and the n+ type layer D3. Now, the p+ type layer D1 and n− type layer D2of the diode DI represent a PN junction diode portion, and the n+ typelayer D3 is a portion provided for connection to the metal electrodeEL3. In the present embodiment, the p+ type layer D1, the n− type layerD2, and the n+ type layer D3 are configured by monocrystalline silicon(Si).

Used as an impurity (acceptor) introduced to the p+ type layer D1 is,for example, boron (B). The p+ type layer D1 has an impurityconcentration of, for example, 3×10¹⁹ cm⁻³. Used as an impurity (donor)introduced to the n+ type layer D3 is, for example, phosphorus (P) orarsenic (As). Diffused in the n− type layer D2 is, for example,phosphorus (P) or arsenic (As). The n-type layer D2 has an impurityconcentration of, for example, 5×10¹⁸ cm⁻³, and the n+ type layer D3 hasan impurity concentration of, for example, 1×10²⁰ cm⁻³.

When monocrystalline silicon is used for the p+ type layer D1, theimpurity concentration of the p+ type layer D1 is preferably set to notless than 3×10¹⁹ cm⁻³. If the p+ type layer D1 has an impurityconcentration less than 3×10¹⁹ cm⁻³, there is a risk that band gapreduction effect does not occur. It is therefore preferable to set tothe above-described impurity concentrations when monocrystalline silicon(Si) is used for the p+ type layer D1.

In the diode DI of the embodiment, the impurity concentration of the p+type layer D1 is higher than the impurity concentration of the n− typelayer D2. Forming each of the layers with such impurity concentrationsresults in a width of the band gap differing between the p+ type layerD1 and the n− type layer D2. A PN junction is formed between the p+ typelayer D1 and the n− type layer D2. The PN junction, i.e., the borderportion between the p+ type layer D1 and the n− type layer D2 is formedby two layers that are formed by the same material, but have differentband gap widths from each other. Accordingly, the PN junction is formedas pseudo-heterojunction.

Operation of this diode DI of the present embodiment is described incomparison to that of a diode DI′ of a comparative example. First, aconfiguration of the diode DI′ of the comparative example is describedwith reference to FIG. 10. FIG. 10 is a cross-sectional view showing astructure of the diode DI′ in accordance with the comparative example.As shown in FIG. 10, the diode DI′ in accordance with the comparativeexample is a PN junction diode comprising a p+ type layer D1′ and an n+type layer D2′. The diode DI′ of the comparative example differs fromthe diode DI of the embodiment in having the n+ type layer D2′ connecteddirectly to the metal electrode. Further, the diode DI′ of thecomparative example has impurity concentrations of impurities introducedto each of the layers that differ from those of the diode DI of theembodiment.

In the comparative example, the p+ type layer D1′ and the n+ type layerD2′ are configured by monocrystalline silicon (Si). Used as an impurity(acceptor) introduced to the p+ type layer D1′ is, for example, boron(B). The p+ type layer D1′ has an impurity concentration of, forexample, 1×10¹⁸ cm⁻³. This is lower than the impurity concentration ofthe p+ type layer D1 of the embodiment shown in FIG. 9. Moreover, usedas an impurity (donor) introduced to the n+ type layer D2′ of thecomparative example is, for example, phosphorus (P) or arsenic (As). Then+ type layer D2′ has an impurity concentration of, for example, 1×10²⁰cm⁻³. This is higher than the impurity concentration of the n− typelayer D2 of the embodiment shown in FIG. 9.

In the diode DI′ of the comparative example shown in FIG. 10, theimpurity concentration of the n+ type layer D2′ is higher than theimpurity concentration of the p+ type layer D1′. Forming each of thelayers with such impurity concentrations results in a width of the bandgap being substantially equal for the p+ type layer D1′ and the n+ typelayer D2′. A PN junction is formed between the p+ type layer D1′ and then+ type layer D2′. The PN junction, i.e., the border portion between thep+ type layer D1′ and the n+ type layer D2′ is formed by two layers thatare formed by the same material and, moreover, have band gap widths thatare equal. Accordingly, the PN junction is formed as homojunction.

Next, operation of the diode DI in accordance with the embodiment andthe diode DI′ in accordance with the comparative example are describedwith reference to FIGS. 11 and 12.

FIG. 11 is a view showing a state of energy bands in the diode DI′ inaccordance with the comparative example. FIG. 11 shows the state ofenergy bands in the PN junction portion, that is, the border portionbetween the p+ type layer D1′ and the n+ type layer D2′ of the diodeDI′. The p+ type layer D1′ and the n+ type layer D2′ are formed bysilicon (Si). Each of the p+ type layer D1′ and the n+ type layer D2′has a band gap of approximately 1.12 eV. As a result, a height of anenergy barrier against an electron current Jn is equal to that against ahole current Jp.

FIG. 12 is a view showing a state of energy bands in the diode DI inaccordance with the embodiment. FIG. 12 shows the state of energy bandsin the PN junction portion, that is, the border portion between the p+type layer D1 and the n− type layer D2 of the diode DI. The p+ typelayer D1 and the n− type layer D2 are formed by silicon (Si). Now aspreviously mentioned, on both sides of the PN junction portion of thediode DI in the embodiment, the impurity concentration of the p+ typelayer D1 is higher than the impurity concentration of the n− type layerD2. When silicon (Si) having such impurity concentrations forms ajunction, a high-concentration band gap reduction effect in the p+ typelayer D1 causes the junction to be a pseudo-heterojunction. The band gapwidth in the p+ type layer D1 is reduced, whereby the energy barrierwith respect to the electron current Jn flowing from the n− type layerD2 toward the p+ type layer D1 is lowered. Consequently, therecombination of electrons and holes is reduced, whereby the forwardbias direction current flowing in the diode DI (current flowing from then− type layer D2 to the p+ type layer D1) increases.

When a diode is configured with the impurity concentrations illustratedin the embodiment, the band gap width is reduced by about 100 meV incomparison with the diode DI′ of the comparative example. Using thediode DI of the embodiment allows the value of the forward biasdirection current to be increased by a factor of 10 or more compared tothe diode DI′ of the comparative example.

[Others]

From the point of view of increasing forward bias direction current in aPN junction diode, it is desirable for the PN junction diode to becompletely crystallized. Hence, description of the embodiment proceededon the assumption that the p+ type layer D1 and the n− type layer D2 inthe embodiment are of monocrystalline silicon. However, the p+ typelayer D1 and the n− type layer D2 are not limited to being ofmonocrystalline silicon.

For example, at least a portion of the p+ type layer D1 and the n− typelayer D2 may be of amorphous silicon or polycrystalline silicon. It ispossible also in the case of using amorphous silicon or polycrystallinesilicon in the p+ type layer D1 and the n− type layer D2 for the forwardbias direction current flowing in the diode DI to be increased similarlyto in the embodiment by configuring the junction interface between thep+ type layer D1 and the n− type layer D2 as a pseudo-heterojunction.There is an increase in the forward bias direction current component dueto the above-mentioned effects of the pseudo-heterojunction, even ifcrystallization of the amorphous silicon or polycrystalline silicon usedin the p+ type layer D1 and the n− type layer D2 is not complete.

As the above makes clear, there is no need for the diode DI to becompletely crystallized in order for the junction interface between thep+ type layer D1 and the n− type layer D2 to be a pseudo-heterojunction.The heat application process for crystallizing the diode DI cantherefore be reduced. As a result, effects of the heating process on thevariable resistor VR can be alleviated, and effects on performance ofthe variable resistor VR can be reduced.

When amorphous silicon is used for the p+ type layer D1, the impurityconcentration of the p+ type layer D1 is preferably set to a range ofnot less than 3×10¹⁹ cm⁻³ and not more than 5×10¹⁹ cm⁻³. If the p+ typelayer D1 has an impurity concentration less than 3×10¹⁹ cm⁻³, there is arisk that band gap reduction effect does not occur, similarly to in thecase of monocrystalline silicon, and if the p+ type layer D1 has animpurity concentration greater than 5×10¹⁹ cm⁻³, there is a risk thatcrystalline defects are generated in the p+ type layer D1. Moreover,when polycrystalline silicon is used for the p+ type layer D1, theimpurity concentration of the p+ type layer D1 is preferably set to arange of not less than 3×10¹⁹ cm⁻³ and not more than 1×10²¹ cm⁻³. If thep+ type layer D1 has an impurity concentration less than 3×10¹⁹ cm⁻³,there is a risk that band gap reduction effect does not occur, similarlyto in the case of monocrystalline silicon, and if the p+ type layer D1has an impurity concentration greater than 1×10²¹ cm⁻³, there is a riskthat the impurity is precipitated in the p+ type layer D1. It istherefore preferable to set to the above-described impurityconcentrations when amorphous silicon or polycrystalline silicon is usedfor the p+ type layer D1.

As shown in FIG. 13, the p+ type layer D1, the n-type layer D2, and then+ type layer D3, provided that they maintain their respectiveelectrical properties, that is, of being p+ type, n− type, and n+ type,may each contain both a p type impurity and an n type impurity. Forexample, the n− type layer D2 may contain a p type impurity. However, inorder to maintain the n− type layer D2 as n− type in this case, aconcentration (Na2) of the p type impurity in the n− type layer D2 islower than a concentration (Nd2) of the n type impurity in the n− typelayer D2. This configuration allows the n-type layer D2 to include boththe n type impurity and the p type impurity, and allows an electrical n−type to be formed by the difference in included amount of these n typeand p type impurities. It is preferable for impurities of bothconductivity types to be included since this facilitates formation ofthe electrically low concentration n− type. Note that, as mentionedabove, it is the fact that the n− type layer D2 is n-type which givesrise to the resulting fact that a pseudo-heterojunction can be formed atthe interface between the p+ type layer D1 and the n− type layer D2.

Similarly, the p+ type layer D1 may contain an n type impurity. In thiscase, a concentration (Nd1) of the n type impurity in the p+ type layerD1 is lower than a concentration (Na1) of the p type impurity in the p+type layer D1. Furthermore, it is preferable for the concentration Nd1of the n type impurity in the p+ type layer D1 to be low in order tomaintain the electrically high concentration p+ type and thereby form apseudo-heterojunction at the interface between the p+ type layer D1 andthe n− type layer D2. Specifically, the concentration Nd1 of the n typeimpurity in the p+ type layer D1 is preferably lower than theconcentration Na2 of the p type impurity in the n− type layer D2.

Likewise, the n+ type layer D3 may contain a p type impurity, in whichcase, a concentration (Na3) of the p type impurity in the n+ type layerD3 is lower than a concentration (Nd3) of the n type impurity in the n+type layer D3. It is preferable for the concentration Na3 of the p typeimpurity in the n+ type layer D3 to be low in order to maintain theelectrically high concentration n+ type. Specifically, the concentrationNa3 of the p type impurity in the n+ type layer D3 is preferably lowerthan the concentration Na2 of the p type impurity in the n− type layerD2.

In the case that the p+ type layer D1, the n− type layer D2, and the n+type layer D3 each contain a p type impurity and an n type impurity, theconcentration distribution of the p type impurity in DI preferably has apeak in the p+ type layer D1 (Na1) and becomes lower in the order of then− type layer D2 (Na2) and the n+ type layer D3 (Na3). Similarly, theconcentration distribution of the n type impurity in DI preferably has apeak in the n+ type layer D3 (Nd3) and becomes lower in the order of then− type layer D2 (Nd2) and the p+ type layer D1 (Nd1). Note that, asshown in FIGS. 9 and 13, a peak value (Nd3) of the n type impurityconcentration is preferably higher than a peak value (Na1) of the p typeimpurity concentration.

Note that measurement of impurity concentration is performed using, forexample, a laser atom probe. A mean value is assumed to be used for theimpurity concentrations in each of the layers.

This concludes description of embodiments of the present invention, butit should be noted that the present invention is not limited to theabove-described embodiments, and that various alterations, additions,combinations, and so on, are possible within a range not departing fromthe scope and spirit of the invention.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

1-9. (canceled)
 10. A semiconductor memory device, comprising: aplurality of first lines disposed in parallel; a plurality of secondlines disposed to intersect the first lines; and a memory cell arrayincluding memory cells, each of the memory cells being disposed at eachof intersections of the first lines and the second lines, and each ofthe memory cells being configured by a rectifier element and a variableresistor connected in series, the rectifier element comprising: a firstsemiconductor region including an impurity of a first conductivity typeat a first impurity concentration; and a second semiconductor regionincluding an impurity of a second conductivity type at a second impurityconcentration lower than the first impurity concentration and includingan impurity of the first conductivity type at a third impurityconcentration lower than the second impurity concentration, the firstsemiconductor region and the second semiconductor region being formed bysilicon.
 11. The semiconductor memory device according to claim 10,wherein the first impurity concentration is not less than 3×10¹⁹ cm⁻³.12. The semiconductor memory device according to claim 10, wherein thefirst semiconductor region includes an impurity of the secondconductivity type at a fourth impurity concentration lower than thethird impurity concentration.
 13. The semiconductor memory deviceaccording to claim 10, wherein at least a portion of the firstsemiconductor region is formed by amorphous silicon.
 14. Thesemiconductor memory device according to claim 13, wherein the firstimpurity concentration is not less than 3×10¹⁹ cm⁻³ and not more than5×10¹⁹ cm⁻³.
 15. The semiconductor memory device according to claim 10,wherein at least a portion of the first semiconductor region is formedby polycrystalline silicon.
 16. The semiconductor memory deviceaccording to claim 15, wherein the first impurity concentration is notless than 3×10¹⁹ cm⁻³ and not more than 1×10²¹ cm⁻³.
 17. Thesemiconductor memory device according to claim 10, wherein the rectifierelement further comprises a third semiconductor region provided to be incontact with the second semiconductor region and including an impurityof the second conductivity type at a fifth impurity concentration higherthan the second impurity concentration.
 18. The semiconductor memorydevice according to claim 17, wherein the fifth impurity concentrationis higher than the first impurity concentration.
 19. The semiconductormemory device according to claim 17, wherein the third semiconductorregion includes an impurity of the first conductivity type at a sixthimpurity concentration lower than the third impurity concentration. 20.The semiconductor memory device according to claim 10, wherein the firstsemiconductor region and the second semiconductor region are formed bymonocrystalline silicon.